San Francisco -LRB- CNN -RRB- -- If somebody pulled out a gun on a crowded train you were riding on , would you notice ? These people did n't .

Why ? They were too into their smartphones , a San Francisco prosecutor says .

The September killing of 20-year-old Justin Valdez on that busy train was shocking enough . The shooter , apparently picking the victim at random , shot the San Francisco State University student in the back .

Also shocking , the prosecutor says , was the initial actions of bystanders . Or inaction .

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`` Some are no more than two to three feet to him , '' said San Francisco District Attorney George Gascon `` We 're seeing people that are so disconnected to their surroundings . This is not unique . People are being robbed , people are being hurt , people are being run over by cars because they 're so disconnected because of these phones . ''

A suspect has been arrested for the killing and pleaded not guilty , authorities said . But the phenomenon of witnesses being distracted by technology may continue , Gascon said .

`` Just for our own safety , would n't you want to know if somebody standing next to you is pulling a gun out ? I think I would , '' Gascon said .

The security footage of the incident is chilling . The man , donning a baseball hat and smile , lifts a .45 - caliber handgun in plain view , three or four times . He waves the weapon as if choosing who he wants to kill . At one point , he even wipes his nose with the gun . But nobody seemed to notice until the blast goes off .

Though this is an extreme and violent example , there are others . You may remember the woman who was so absorbed with texting that she fell right into a mall fountain . Or you remember the guy who was so engrossed in his phone he almost walks right into a black bear . But those were just harmless viral videos that many have laughed at .

This is different , especially for San Francisco State University students .

`` That 's just people 's stupidity , I guess , ignorance of what 's happening around them , '' said Whitney Bulmer .

And there is no sign that this will end . The other day at the same train stop where Valdez boarded for the last time , a group of college students milled around , most of them too focused on their phones to notice a CNN camera focused on them .

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A college student was killed on a train in San Francisco

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DA : The gunmen waved his gun around and showed it before the shooting

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Nobody noticed the gun before the shooting , DA says